1. Field of the Invention
The present invention is about a method and apparatus for improvement of vertical tube evaporation (VTE), and particularly, an improved method and means or a system for the distribution of liquid feed with or without solids suspended therein, or wherein liquid feeds are distributed with some vapor as mixed liquid and vapor feed into a plurality of vertical evaporation channels and for liquid flashdown into a plurality of evaporator units operated in series, or in a series of effects, and particularly with the objective of controlling the rates of cascading feed transfer from effect to effect in such a multi-effect (ME) series of downflow evaporators, and to control their effect-to-effect interactions including continuous or adequate feed flow or transfer into all the effects, and to assure continuous and adequate feed flow into all the tubes of such a multi-effect vertical tube evaporator (ME-VTE) in continuous operation, and to improve the overall effectiveness or thermal efficiencies of a series of such evaporator units operated in a multi-effect series, and especially applicable to a vertically stacked, stepped or staggered series of downflow evaporators wherein effect-to-effect feed pumps are not used or not needed. Feed flow is preferably downward in such a series, cascaded or added to an effect or to the next effect by flow subject to gravity.
Conventional vertical tube evaporators (VTE) are usually operated in a series of side-by-side multiple effect (ME) units, wherein the feed liquid is transferred from one effect to the next effect by a pump for the downflow VTE mode and wherein a portion of this pumped feed may be recycled into the top of the same effect to maintain an adequate feed flow there-into. In the upflow VTE mode of ME operation, the feed is transferred by the effect-to-effect static head available, which is at least about equal to the length of such upflow effect VTE tubes, and by means of vapor lifting the liquid through the tubes. Conventionally, heat in the form of steam or vapor and hot liquids, including warm brine, condensates or distillates, are transferred along the series of effects in a manner that improves the amount of product distillate obtainable per amount of heating steam added at the hot end of the ME series, and to drive the evaporation process. Such a multiple effect (ME) VTE series provides for increasing the performance ratio or the economy ratio or the number of pounds of distillate produced per pound of boiler steam applied; and this ratio can be about equal to the number of effects used. In the downflow mode, conventional effect-to-effect feed transfer and flowrate into the top ends of such evaporator tube bundles depend on a feed pump, and the feed distribution into the tubes depends on pump pressure and on nozzles or orifices used. Significant cost advantages can result from replacing the effect pumps by either using the upflow mode of feed flow and feed transfer or by the vertical stacking of the effects and by using the downflow mode as herein applied. In the absence of effect pumps and recycled feed within an effect, inadequate feed flow, dryout of tubes and scaling or hydrodynamic instabilities can affect evaporation performance adversely; this disclosure overcomes or mitigates these problems.
2. Description of the Prior Art References
Downflow and upflow vertical tube foam evaporation (VTFE) was disclosed in U.S. Pat. Nos. 3,846,254 and 4,511,432. Dispersed seeded slurry evaporation (DSSE), also called dispersed slurry evaporation (DSE) was disclosed Ser. No. 07/187,817 filed Apr. 29, 1988 and issued as U.S. Pat. No. 5,156,706.
U.S. Pat. No. 3,846,254 is about vertical tube foamy flow evaporation (VTFE) wherein the thermal efficiency for both the downflow and upflow modes of VTFE operation is substantially increased by imposing a foamy liquid layer flow within the vertical tubes by the addition of selected foaming agents. Single, fixed-orifice plates were used for feed distribution in both modes of operation.
In U.S. Pat. No. 4,511,432 a system for feed distribution into a multiplicity of parallel upflow evaporator tubes was disclosed to improve upflow two-phase flow stability and to improve the performance of such an upflow evaporator. The main objective was to control upflow vapor-liquid stability by means of an orifice plate spaced from the tube bundle inlet ends, and wherein this vertical spacing is adjustable, and is used to prevent oscillating cross flow within this space and amongst the evaporator tubes within an evaporator body or effect. The gap provided between this single orifice plate and the tube inlet ends was reduced to a minimal size to overcome unstable flow in the upflow evaporator tubes. To control this gap adjustment, the tubeside pressure drop was monitored and used as a signal to drive the mechanism for adjusting the gap or the vertical space between the single orifice plate and the tube inlet ends. The vapor-liquid upflow stability was thereby controlled, but not the rate of flow through the orifice plate. This reference is about upflow evaporators mounted side-by-side, and whereby transfer of the feed liquid from one effect to the next is driven by a hydrostatic head which is in direct relationship to the vertical distance between the tube outlet ends of the upstream effect and the tube inlet ends of the downstream effect, or approximately equal to a column of liquid as high as the effect tube length. The spacing or gap between this single orifice plate and the tube inlet ends is variable, and is used to control upflow stability.
In another reference, U.S. Pat. No. 5,156,706, a method and apparatus were disclosed for improving feed distribution with a single orifice plate into a multiplicity of parallel tubes mounted in a downflow evaporator, and the feed is supplied by a feed recycle pump, or is under pump pressure. The feed is distributed through orifices in a single plate and of essentially equal flow diameter, at essentially constant flow rates into the tube inlet ends. All the orifices disclosed are in single plates, all of the plates are fixed except one which is mounted to be vertically movable to adjust the gap or spacing between this plate and the tube inlet ends. Vertical cylindrical attachments to the underside of two plates shown extend into the tubes as feed deflectors, and form an annular area for deflecting the feed onto the tube inner walls for annular flow. In one instance a tubular attachment to the orifice plate and extending into the tube may be tapered and forms a variable annular opening with the tube wall to provide variable annular feed flow rates. The present patent application is a CIP of a CIP of a CIP which copended with Ser. No. 187,817 which issued as U.S. Pat. No. 5,156,706 on Oct. 20, 1992.